docs: def to_child_field(cls):
      """
      Returns an callable instance that will convert a value to a Child object.
  
      :param cls: Valid class type of the Child.
      :return: instance of ChildConverter.
      """
  
      class ChildConverter(object):
  
          def __init__(self, cls):
              self._cls = cls
  
          @property
          def cls(self):
              return resolve_class(self._cls)
  
          def __call__(self, value):
              try:
                  # Issue #33: if value is the class and callable, then invoke
                  if value == self._cls and callable(value):
                      value = value()
  
                  return to_model(self.cls, value)
              except ValueError as e:
                  error_msg = CHILD_ERROR_MSG.format(value, self.cls, str(e))
                  raise ValueError(error_msg)
  
      return ChildConverter(cls)